Longest Consecutive Sequence

题目：

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example,
Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

Your algorithm should run in O(n) complexity.

分析：

用一个set存储集合中的所有元素，

然后遍历数组的时候，查找set中是否有当前元素+1，-1之后的元素，同时删除掉；

因为连续的元素会在一次当中删除掉，进而得到这个连续段的最大长度，或者说这个连续段不论从哪个元素开始，得到的最大长度都是一样的，所以查找一次就够了！！

反之，其他连续段中的元素，这次查找过程是查不到的，也是删不掉的！

参考代码如下：

http://www.programcreek.com/2013/01/leetcode-longest-consecutive-sequence-java/

public class Solution {    public static int longestConsecutive(int[] num) {    // if array is empty, return 0    if (num.length == 0) {    return 0;    }         Set<Integer> set = new HashSet<Integer>();    int max = 1;         for (int e : num)    set.add(e);         for (int e : num) {    int left = e - 1;    int right = e + 1;    int count = 1;         while (set.contains(left)) {    count++;    set.remove(left);    left--;    }         while (set.contains(right)) {    count++;    set.remove(right);    right++;    }         max = Math.max(count, max);    }         return max;    }}